The present invention relates generally to the production of a voltage signal and more particularly to the synthesization or simulation of the equivalent voltage of a supply, which supply need have only a pair of output terminals accessible.
There are many instances in which it is desirable to provide a signal; e.g., for feedback purposes, which is proportional to or which duplicates a supply voltage. Usually, the production of these signals presents no problem as any voltage sources within the supply are readily accessible and there are many ways by which suitable reproduction of this available voltage(s) can be achieved. There are, however, certain instances in which the reproduction of such a voltage does represent a problem. Examples include situations in which the voltages are very high or mechanical restrictions prevent access to the voltage source(s). Another example is that situation in which the effective voltage, desired to be reproduced, is derived from a plurality of individual sources which combine in a manner which make it impractical or impossible to directly access and combine the individual sources. In a sense, these situations are those in which there exists a "black box" without reasonable access to the interior and a need to provide, even during periods when the effective voltage of the circuitry within the box is changing, a simulation of the equivalent output voltage of that box. This equivalent voltage is known in the art as the Thevenin equivalent voltage.
A specific example of an application in which high voltages, mechanical restraints and plurality of voltage sources preclude a direct reproduction of the effective voltage may be found in certain generator excitation systems. One known type of generator excitation system, commonly referred to as "self-excited," employs voltage control of the field winding by permitting the short-circuiting of the field winding for a percentage of the time during each of the normal operating cycles of the generator to thereby control the degree of excitation. An example of such an excitation control system may be found in U.S. Pat. No. 3,369,171, "Control Circuits" by L. J. Lane, issued Feb. 13, 1968 and assigned to the assignee of the present invention. In this patent the generator field winding is excited through the use of the diode bridge circuit which receives power in the form of two voltage signals which are, respectively, proportional to the generator terminal voltage and to the current supplied to the generator load. In this patent, that controlled is the generator terminal voltage.
A known modification of the system of the U.S. Pat. No. 3,369,171 patent utilizes a feedback signal to control the field voltage. This system is outlined in greater detail in co-pending patent application, Ser. No. 607,784, "Generator Control System" by G. S. Chambers and L. J. Lane, filed on even date herewith and assigned to the assignee of the present invention. As explained in that co-pending application, the prior art control of the U.S. Pat. No. 3,369,171 exhibits a nonlinearity in the feedback path due to a multiplication which is inherent in the circuitry so that the control is not as accurate as desired in all instances. Accordingly, in the co-pending application there is provided an improved system wherein the Thevenin equivalent voltage of the exciter bridge circuit is utilized as a modifying factor to linearize the feedback control. As explained in that co-pending application, the improved generator excitation system requires, for an enhanced feedback control, an accurate representation of the equivalent voltage, i.e., the Thevenin equivalent voltage, of the exciter circuit and, as stated therein, preferably employs the present invention to achieve that equivalent voltage.